Input and output devices for optical fiber

ABSTRACT

Input and output devices for an optical fiber include an acoustic transducer for generating a planar acoustic wave tilted at an angle with respect to the axis of an optical fiber embraced by a quartz block. The gap between the optical fiber and the quartz block is filled with a liquid, such as water, to obtain acoustic impedance matching. A desired mode of propagating light can be extracted from the fiber or injected therein by reflection under Bragg&#39;s condition through a glass block and optical system without cutting or damaging the optical fiber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to acoustic input and output devices for alight conducting optical fiber.

2. Description of the Prior Art

In a conventional input device for an optical fiber, an input light beamis usually introduced into the fiber through its end face eitherdirectly or by way of an optical lens system. Such a conventional devicesuffers from the obvious drawback that the input light beam can only beinjected into the optical fiber through an end face, and not at anyintermediate location.

There is also a requirement for an optical fiber output devicefunctioning as a mode analyzer or monitor. Two kinds of such outputdevices are known in the art. The first one extracts a propagating lightbeam by cutting a portion of the optical fiber where the output iswanted. Such a device is inconvenient since an output light beam cannotbe extracted without cutting the optical fiber. In a second such devicethe optical fiber is machined down to a tapered shape at a desiredlocation to implement the extraction of the propagating light beam. Inthe device the machining is difficult and troublesome, and the opticalfiber cannot thereafter be restored to its original state.

Some correlated prior art in this technical field is listed below:

1. J. E. Fulenwider, U.S. Pat. No. 3,871,743 issued on June 4, 1973,

2. W. V. Smith, "IBM Technical Disclosure Bulletin" 14 No. 2, July 1971,p. 652,

3. E. G. Lean et al, U.S. Pat. No. 3,791,715 issued on Dec. 11, 1972,and

4. R. Ulrich, U.S. Pat. No. 3,905,676 issued on Sept. 16, 1975.

SUMMARY OF THE INVENTION

The present invention mitigates the aforementioned drawbacks of theprior art by providing input and output devices for an optical fiberwhich can be used to inject or extract a desired guided mode or modes ofa propagating light beam at any location on the optical fiber withoutcutting or machining the fiber. Briefly, and according to the invention,input and output devices for a optical fiber include an acoustictransducer for generating a planar acoustic wave tilted at an angle withrespect to the axis of an optical fiber embraced by a quartz block. Thegap between the optical fiber and the quartz block is filled with aliquid, such as water, to obtain acoustic impedance matching. A desiredmode of propagating light can be extracted from the fiber or injectedtherein by reflection under Bragg's condition through a glass block andoptical system without cutting or damaging the optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1a and FIG. 1b are diagrams for explaining two phase matchingconditions among the wave vectors of the guided mode, radiation mode andthe acoustic wave transmission,

FIG. 2 is a diagram for explaining a relation between the normalizedfrequency V and the coupling constant ratio C₂ /C₁,

FIG. 3 is a perspective view showing one embodiment of an output devicefor an optical fiber made according to the present invention,

FIG. 4a and FIG. 4b are enlarged partial cross-sectional andlongitudinal-sectional views of the output device shown in FIG. 3,

FIG. 5 is a diagram showing the relation among the wave vectors of theguided mode, the acoustic wave and the scattered light in a multi-modeoptical fiber,

FIGS. 6 to 11 are diagrams for explaining various characteristics of anoptical fiber,

FIG. 12 is a perspective view of an output device for an optical fiberusing a converging type of acoustic transducer according to the presentinvention, and

FIG. 13 is a perspective view of an embodiment of an input device for anoptical fiber according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to enable a clear understanding of the present invention,embodiments thereof will be explained by referring to the attacheddrawings.

Initially, a basic principle of the interaction between an acoustic waveand a light beam in an optical fiber will be explained. An optical fiberconsists of core glass at the center and clad glass at thecircumferences. However, for simplification, only a slab or plate typeof optical path, in which a plate shaped core of glass is sandwichedbetween two clade plates, will be considered. In such a slab typeoptical path, it is sufficient to consider only the influence in twodimensional propagation. The interaction is analyzed by means of acoupled mode equation and the phase matching condition is obtained fromthe solutions thereof. From the solutions it has been found that twosuch phase matching conditions exist as shown by the vector diagrams ofFIGS. 1a and 1b. In such phase matching conditions, the lightpropagating in a guided mode is strongly reflected at the core surfaceand changes to a radiation mode.

If we assume λ, n₁, and n₂ to be the wavelength of the transmittinglight in vacuo, the refractive index of an optical fiber core and therefractive index of its clad plates, respectively, then the wave numberk₁ of a plane shaped light wave in the core and the wave number k₂ of aplane shaped light wave in the clad may be expressed as follows.

    k.sub.1 = 2π/λn.sub.1                            (1)

    k.sub.2 = (2π/λ)n.sub.2                          (2)

FIG. 1a corresponds to a term obtained from the calculation of theinteraction in the core. This term satisfies the so-called Bragg'scondition as follows:

    k.sub.1g + K = k.sub.1r,                                   (3)

wherein k_(1g) is a wave vector of the light in the core in the guidedmode, k_(1r) is a wave vector of the light in the radiation mode, and Kis a wave vector of the acoustic wave.

Similarly, FIG. 1b corresponds to a term derived from the calculation ofthe interaction in the clad portion. This phase matching condition isgiven by the following equation:

    β + K = k.sub.2r,                                     (4)

wherein k_(2r) is a wave vector of the light in the radiation mode inthe clad portion and β is an orthogonal projection to the fiber axis ofthe wave vector k_(1g) in a guided mode in the core. If we term the wavevector of the light in a guided mode in the clad to be β, then equation(4) may be regarded as the Bragg's condition or rule in the cladportion.

FIG. 2 is a diagram for showing which of two phase matching conditionsas shown in FIGS. 1a and 1b prevails in practice. FIG. 2 shows arelation between a ratio of two coupling constants C₂ / C₁ and thenormalized frequency V, wherein C₁ is a coupling constant between theguided mode and the radiation mode when the phase matching conditionshown in FIG. 1a is satisfied, and C₂ is that when the condition shownin FIG. 1b is satisfied.

The value of the normalized frequency V, hereinafter referred to as theV number, in an optical fiber path is given by the following equation:

    V = (2πa/λ)√n.sub.1.sup.2 -n.sub.2.sup.2, (5)

wherein a is radius of the fiber core and λ is the wavelength of thelight propagating in vacuo. This V number of the normalized frequency isa parameter for defining the construction of the optical fiber. In thisrespect reference is made to the text by Kapany and Burke entitled"Optical Waveguides"; Academic Press.

In a region in which the V number is less than 1.57, only a single modeof propagation exists. This region is referred to as a single moderegion. When the V number increases above 1.57, a greater number ofmodes appear in proporation to the increase of the V number.

In FIG. 2, a horizontal dotted line passing through the point where C₂/C₁ =1 represents a portion wherein the phase matching conditions shownin FIGS. 1a and 1b contribute up to the same extent. Above the dottedline, wherein C₂ /C₁ >1, is a region in which the conditions shown inFIG. 1b prevail. Below the dotted line, wherein C₂ /C₁ <1, is a regionin which the conditions shown in FIG. 1a prevail.

From FIG. 2, in the single mode region, wherein V is less than 1.57, thephase matching condition as shown in FIG. 1b is to be considered sincein that region C₂ /C₁ is larger than 1 (C₂ /C₁ >1). In the multi-moderegion, wherein V is larger than 1.57, i.e., on the right side ofV=1.57, higher modes appear successively according to an increase in theV number. However, most of the higher modes pass the region where C₂ /C₁<1. Therefore, as a close approximation only the phase matchingcondition shown in FIG. 1a may be considered.

The invention will be explained at first by describing an example of anoutput device for a multi-mode optical fiber made in accordance with thepresent invention which may be used as a mode analyzer. According to theprevious explanation it is sufficient that only the condition shown inFIG. 1b be taken into account in an output device for a multi-modeoptical fiber.

FIG. 3 is a perspective view of an output device for an optical fiber,and FIGS. 4a and 4b are partially enlarged cross-sectional viewsthereof.

Referring to FIG. 3, an acoustic transducer 10 is mounted on an inclinedsurface 12 of a quartz block 14 having trapezoidal side surfaces 16 andrectangular end surfaces 18 and 20, as best seen in FIGS. 4a and 4b. Thequartz block 14 is provided with a groove 22 having nearly the sameradius as an optical fiber 24 through which the light, whose modedistribution will now be analyzed, is transmitted in a direction fromleft to right as shown by the arrow Z. The optical fiber 24 has a coremade of a fused quartz having the refractive index, for instance, of1.46. In actual practice the optical fiber is very thin. For instance,the core diameter is on the order of 100 μm. In FIGS. 4a and 4b theoptical fiber is shown greatly enlarged for the purpose of explanation,and it should be noted that these figures are not drawn on an exactlyproportional scale. A glass block 30, also having trapezoidal sidesurfaces and a narrower end face in the same side with that of thequartz block 14, is arranged so as to embrace the optical fiber 24 inits groove 34 together with the quartz block 14. This glass block 30 ismade of an optical glass having a higher refractive index than that ofthe glass fiber 24. As mentioned above, if the refractive index of theglass fiber is chosen to be 1.46, the refractive index of the glass usedfor the glass block 30 may be 1.51 for a light beam having a wavelengthof 6328 A.

An example of such an optical glass is known under the tradename of BK7.

An optically transparent sheet 26 having a higher refractive index thanthat of the optical fiber 24, for example, a polyethylene sheet having arefractive index of 1.476, is arranged in the groove 34 of the glassblock 30 and around the lower half surface of the optical fiber 24.

Matching liquid 28 for the acoustic wave is introduced between theoptical fiber 24 and the quartz block 14 by capillary action. Thismatching liquid may be water, which has a less absorbing effect for theacoustic wave and has less reflection loss at the boundary surfacesbetween the quartz block 14 and the optical fiber 24 by reason of thesmall difference of the acoustic impedance.

A slit 38 is provided substantially parallel with the inclined bottomsurface 36 of the glass block 30 and aligned with output optical beam.The slit 36 is defined by a plate member supported by a suitable supportmeans 40. A converging lens 42 is provided below the slit 38 and issupported on a suitable support means 44. A photo detector 46 issupported by a support means 48 around the focal point of the lens 42.

By arranging the radius of the groove 22 in the quartz block 14 to benearly equal to the radius of the optical fiber 24, or by arranging theclearance to be very small, the lens effect for the acoustic wave in thewater 28 therebetween can be made very small even though the velocity ofsound in the water is different from that in the quartz block 14 and inthe optical fiber 24. Thus, the plane wave component of the acousticwave derived from the acoustic transducer 10 can be introduced into theoptical fiber 24 without any substantial change of such plane wavecomponent.

the abovementioned acoustic matching is a very simple technique, and itis possible to obtain outputs in each different direction according tothe respective guided modes. This will be explained hereinafter. Thegroove 22 in the quartz block 14 having a radius on the order of 100 μmcan be manufactured by drawing a thick quartz tube to make the innerhole very small and by cutting the thus drawn thin tube.

In operation, the acoustic transducer 10, which may consist of a bulkbody of LiNbO₃ or a thin film of ZnO, is excited with a high frequencyelectric voltage to generate an acoustic wave in the ultrasonic range.As can be seen from FIG. 4b, the acoustic transducer 10 is secured onthe top surface of the quartz block 14 which is inclined at an angle αto the axis of the optical fiber 24. The acoustic wave, generated by thetransducer 10 and incident upon the optical fiber 24, is also inclinedat an angle α to the axis thereof. This angle α can be set at a desiredvalue, which will be explained hereinafter, by suitably selecting thetilt or inclination angle α of the quartz block 14. Each of the guidedmodes of the light beam passing through the optical fiber 24 arereflected in the fiber by the acoustic wave according to Bragg'scondition, and a part or whole thereof is emanated down through theglass block 30. The portion of the light passing through the slit 38 iscollected by the lens 42 and detected by the photo detector 46.

By changing the frequency of the acoustic wave generated by thetransducer 10, each of the guided modes of the light beam passingthrough the optical fiber 24 can be reflected out in succession. Theoutput light beams may be detected by the photo detector 46, which maybe an avalanche photo-diode, and the modes may thereafter be suitablyanalyzed.

In an alternate embodiment, the frequency of the acoustic wave is fixedin the device shown in FIG. 3, but by spatially moving the slit 38 andthe lens 42, the various modes propagated in the optical fiber 24 may beread out in succession. In this case, the modes may also be sensed bythe photo detector 46. The mode detection may also be effected byplacing a photographic film at the slit 38 and taking a far fieldpattern photograph of the scattered output light.

The guided modes in the core of the clad type optical fiber core havinga propagation constant β can be expressed by the following equations:##EQU1## or ##EQU2## wherein; Jm is mth order Bessel function,

r and φ are positional coordinates expressed in the cylindricalcoordinate system,

z is a function expressing the direction of the optical fiber,

m is the number of modes at a given angle direction,

a is the radius of the core,

ω is the angular frequency of the light, and

t is time.

For a more detailed development, refer to the text by D. Marcuseentitled "Theory of Dielectric Optical Waveguide"; 1974, Academic Press.

Further, if we assume the wave number of the plane wave of the lightbeam in a core is k₁, the normalized propagation constant u in theradial direction is given by the following formula:

    u.sup.2 = a(k.sub.1.sup.2 =β.sup.2).

By expanding the above equation of the guided mode, whose propagationconstant is β, by using a plane wave, the following equations arederived for multi-mode transmission in an optical fiber: ##EQU3## or##EQU4## For more detail refer to the text by P. M. Morse and H.Feshback entitled "Method of Theoretical Physics"; 1953, McGraw Hill.

The principle of the invention will now be further explained byreferring to FIG. 5. FIG. 5 shows the relation among the wave vectors ofthe guided modes, including the acoustic wave and the scattered waves.In FIG. 5, the direction of z represents the axial direction of theoptical fiber. According to equations (8) and (9), the guided modes,having the propagation constant β, can be expressed by the components ofwave vectors originating from an origin 0 andarriving at points on acircular line AA' having its center on the axis z and a radius of k₁·sinθ. Here the propagation angle θ is defined by the followingequation:

    sinθ = (u/ak.sub.1).

This means that the components of the wave vectors are limited to lie ongenerating lines of a right circular cone OAA'.

A plane acoustic wave incident upon the optical fiber has a certain tiltangle α with the axis of the fiber. Consider now a condition where anacoustic wave having a wave vector K is introduced into the opticalfiber under a geometrical relation shown in FIG. 5, in which a wavevector OA plus K equals OB. According to equation (3) this conditionmeans that Bragg's condition is satisfied, or in other words, thereflected beam toward the direction of the vector OB is stronger thanthe diffused light beam in any other direction.

Since there exists a condition that:

    |K| = (2πf/W),

wherein f is the frequency of the acoustic wave and W is the velocity ofthe acoustic wave, the light beam can be taken out by making theacoustic frequency f or the absolute value of the wave vector K so largethat the vector OB will not become a guided mode. The desired conditionfor the frequency f may be given by the following equation: s ##EQU5##In this case, the tilt angle α is given by the following:

    α = θ sin.sup.-1 (πf/k.sub.1 W).            (11)

by observing the light thus taken out, the light intensity for the modehaving a propagation constant β can be presumed. In this method, themode of light for satisfying Bragg's condition with the wave vector K isnot only the vector OA, but all vectors connecting the origin O with anypoint on the circle AC. For instance, vector OG satisfies Bragg'scondition, and reflected or scattered vectors connect the origin O andthe circle BD. Here, both the circles AC and BD are on parallel planespassing through the points A and B, are normal to the wave vector K, anda sphere having an origin O and a radius k₁. The modes having apropagation coefficient of β or less than β are all taken out orextracted from the optical core. Furthermore, as can be clearly seenfrom FIG. 5, the direction of the light reflected from the optical fiberby Bragg's condition differs depending on the value of β.

If all the modes are to be taken out, it is necessary to make the lowestmode reflected to a radiation mode. Referring to FIG. 5, this means thatthe wave vectors must satisfy the following equation:

    OE + K = OM.

accordingly, from this equation, the necessary acoustic frequency f isgiven as follows:

    f = (Wn.sub.1 /λ)√2δ,                  (12)

wherein W is the sonic velocity of the acoustic wave and δ is a relativerefractive index difference and is given as follows:

    δ = (n.sub.1 =n.sub.2 /n.sub.1).                     (13)

If we introduce in equation (12) the value of the sonic velocity in thefused quartz for W and the value of the refractive index of fused quartzfor n₁, the frequency f varies as shown in FIG. 6 against the value ofδ. In practice, the value of δ is between 0.05% to 1.0%, so that thefrequency range of 270 MHz to 1,200 MHz may be used. The tilt angle α ofthe acoustic transducer is given by the following equation forsatisfying Bragg's condition:

    α = 2sin.sup.-1 (2πf/2k.sub.1 W).                 (14)

fig. 7 shows the relation between the tilt angle α and the acousticfrequency f for a multi-mode optical fiber.

FIG. 8 shows the reflected angle for each mode in the core when acousticwaves of 390 MHz, 675 MHz and 870 MHz are successively radiated ontoglass fibers whose relative refractive index differences δ are 0.1%,0.3% and 0.5%, respectively, The light is refracted when it leaves thecore and enters the clad, so that the emanating angle of the light fromthe clad becomes smaller.

As has been explained with reference to FIGS. 3 and 4a, a transparentsheet 26 is inserted between the optical fiber 24 and the glass block30. The transparent sheet has a higher refractive index than that of theoptical fiber 24 and is made of, for example, polyethylene. By theprovision of the transparent and higher refractive index sheet and bymaking the refractive index of the glass block 30 higher than that ofthe optical fiber 24, the scattered light can be taken out. If therefractive indices of the transparent sheet and the glass block arelower than that of the glass fiber, the light arriving at the boundaryof the clad is totally reflected back into the optical fiber and cannotbe taken out, though the guided mode is converted to the radiation modeby Bragg's condition.

The incident angle resolution Δγ under Bragg's condition is determinedby the following formula: ##EQU6## (For the more detail, refer to thetext by Uchida and Niizeki entitled "Acoustic Deflection Materials andTechniques"; Proc IEEE Vol. 61, p. 1073, 1973) wherein L is the lengthof the acoustic transducer, and γ_(B) is Bragg's angle as given by thefollowing: ##EQU7##

The relation between the resolution angle Δγ and the acoustic frequencyf (MHz) is shown in FIG. 9. Compared to the allowable guide angle of anoptical fiber which is on the order of a few degrees, the resolution forfrequencies over 10 MHz, which itself is less than 2° according to FIG.9, is increasingly smaller. Accordingly, for frequencies over 10 MHz thedevice is well suited as an output device for a mode analyzer.

When Bragg's condition is satisfied, the intensity of Bragg reflectionis in proportion to sin² (k₁ ΔnL/2), (Refer to Uchida and Niizeki asabove) wherein Δn is the variation of the refractive index due to anacousto-optical effect and is given by the following:

    Δn = -√M.sub.2 P.sub.A /2.                    (16)

here, P_(A) is the power density of the acoustic wave and M₂ is a figureof merit of the acousto-optical effect determined by the material used.

When fused quartz is used as the optical fiber, and if we assume thatthe interaction between the light beam and the acoustic wave is effectedin an area of 10 mm × 0.1 mm, then about 6.8 W of acoustic wave power isrequired for obtaining 100% reflection.

In actual practice, however, using a 10 mm = 0.1 mm acoustic transducer,a quartz block of 1 mm width, an optical fiber having a 100 μm radius,water between the optical fiber and the quartz block having clearance of50 μm, and an input power of 6.8 W to the acoustic transducer in thedevice as substantially shown in FIG. 3, the ultrasonic acoustic wavemay be damped by reflection, absorption or refraction during itstransmission and hence a greater input power may be needed. The abovelosses have a frequency dependent nature and the output efficiency η in(%) plotted against the acoustic wave frequency f (MHz) is shown in FIG.10. In the above it has been assumed that the conversion efficiency ofthe transducer is 100%.

Considering now a situation in which the lowest order mode, or the modehaving a propagation angle θ=O, just satisfies Bragg's condition with awave vector K₀ in FIG. 5, the situation may be expressed as follows:

    OE + K.sub.0 = OF.

in this case, Bragg's condition for the mode having a propagation angleθ becomes:

    OA + K = OB.

since the transducer is fixed, K is parallel to K₀. Accordingly, byvarying only the absolute value of K,i.e., varying |K|, to obtain theoutput of all the modes it is necessary to make the propagation angle θof the reflected light OF of the lowest guided mode OE larger than2θ_(m), wherein θ_(m) is the propagation angle of the highest guidedmode. By obtaining the above condition, both OF and OB become radiationmodes and the light beam of each mode may be read out in succession byvarying the absolute value of K.

In other words, by fixing the angle α at any convenient value overθ_(m), the following can be obtained from equation (11):

    f = k.sub.1 W/π)sin(α-θ).                   (17)

This means that by varying the frequency f of the acoustic wave,different guided modes having different propagation angles θ in theguided mode can be read out. In this case, the frequency f from equation(17) may be changed from k₁ W/πsinα to k₁ W/πsin(α-θ_(m)) and thepropagation angle θ can be changed from 0 to θ_(m).

When we consider an optical fiber having a difference between therefractive indices of the core and clad δ of 1% (or as a second example0.1%), the angle θ_(m) is about 8.1° (2.6°), and the maximum frequencyf_(m) of the acoustic frequency f should be greater than: ##EQU8## Thismeans that the maximum frequency f_(m) should be more than 2.5 GHz (780MHz). The transducer may be mounted at a tilt angle α of 8.1° (2.6°)relative to the axis of the optical fiber, as seen from FIG. 7, when theacoustic frequency f is 2.5 GHz (780 MHz). The frequency f may be variedover a range of 2.5 GHz (780 MHz). By varying the frequency f of theacoustic wave in such a manner, it is possible to read out all of theguided modes of the light beam passing through the optical fiber byvarying only the absolute value |K| of the wave vector K. Furthermore,if the device is used in an input mode, which will be explained later,it has the advantage of being able to introduce all of the guided modes.

In a further embodiment of the present invention, the frequency of theacoustic wave produced by the acoustic transducer is so selected thatthe propagation angle of the reflected light of the lowest order guidedmode light transmitted in the optical fiber, by Bragg's condition,becomes larger than the propagation angle θ_(m) of the highest orderlight being transmitted, and thus various guided modes can be read out.This means that the frequency f of the acoustic wave and the tilt angleα are selected for a value of θ=0 in equations (10) and (11).

As an example, the frequency f of the acoustic wave may be fixed at asuitable value higher than 1.23 GHz (or as a second example 390 MHz) foran optical fiber whose relative refractive index difference δ is 1%(0.1%), as can be seen from FIG. 6. The tilt angle α of the acoustictransducer is adjusted to satisfy Bragg's condition for the lowestguided mode at the above frequency, and thus all of the various modescan be read out. In one example, a 1.5 GHz transducer is fixed at a tiltangle of 4.9° and in another example a 400 MHz transducer is fixed at anangle of 1.3°. For a plot of the tilt angle α as a function oftransducer frequency, refer to FIG. 7.

The reflected light of each mode is emanated in a different direction,so that each mode may be read out in succession by moving the lenstogether with the slit.

If the device is used as a mode analyzer, the slit and the lens may beremoved and a photograph of the far field pattern may be taken at theslit position. The advantage of obtaining the outputs of all of theguided modes is the same as with the previous example in which theabsolute value |K| of the wave vector K is adjusted.

As has been explained above, the device according to the presentinvention may be used as a mere optical monitor. In this case, only apart of the transmitted light beam may be read out at an arbitrarylocation of the optical fiber without altering or affecting the guidedmodes of the light beam.

As further mentioned above, it is advantageous to read in or extract thelight beam using the phase matching condition shown in FIG. 1b in thecase of a single mode optical fiber. The phase matching condition whenexpressed by equation is as follows: ##EQU9## In this equation, thesymbols have the same meaning as mentioned before. If the tilted angle αof the transducer is set in a suitable range to satisfy the condition of(βcosα/k₂)≦1, then the frequency f is determined as a function of β.Thus, by adjusting the frequency f of the acoustic wave to satisfyequation (18) for a desired guided mode, such mode of light may be readout.

When the β value of a single mode fiber is unknown, it can be determinedby observing the change of the output light intensity as the frequency fis varied. When the output light intensity becomes maximum at a certainvalue of frequency f, then the β value may be calculated from equation(18).

An embodiment having a tilt angle α of 45° will now be considered. Therelation between β and f is shown grafically in FIG. 11. In the abscissathe value of β normalized by k₂, i.e., β/k₂ is plotted. The guided modeis in the region of β/k₂ >1. The value of the relative refractive indexdifference δ of an optical fiber in practice lies in the region aroundor less than 1%, so that β/k₂ may be less than about 1.01.

By calculation, the frequency f of the acoustic wave for reading out thelight is less than about 100 MHz, and is therefore in a comparativelylow frequency range. Furthermore, since β and f are monotonousfunctions, by varying the value of f between 0 and 100 MHz and notingthe value of f at which the maximum intensity light is read out, it ispossible to determine the propagation constant β of the guided mode andthus the device may be used as a mode analyzer for a single mode.

In the foregoing explanation, the acoustic transducer is plate shapedand a planar acoustic wave is generated. However, by using an acoustictransducer 50 having a curved surface as shown in the embodiment of FIG.12, it is possible to further concentrate the acoustic power in theoptical fiber, whereby greater efficiency can be obtained.

If the device is to be used in an input mode, the ultrasonic acousticwave is generated in the same manner as explained above and the inputlight beam is fed into the device from reverse direction of theemanation of the output light. The light beam is reflected by theacoustic wave, according to Bragg's condition, and it becomes a part orthe whole of the guided mode in the optical fiber. The direction ofpropagation in the optical fiber is opposite to that of the outputdevice. An electromagnetic wave is reversible with respect to time, sothat if it is possible to make the wave front of an incident light beamcoincident with that of an output light beam but travelling in thereverse direction, the relation between the incident light and theguided mode will be the same as the relation between the output lightand the guided mode, as developed above.

As an exemplary embodiment of an optical system for generating anincident light beam having a reversely propagating wave front withrespect to that of an output light beam, an input optical device using ahologram coupler will now be described with reference to FIG. 13, whichshows a simplified perspective view of such an input device. At firstthe manner of producing an input hologram will be explained. By applyinga light beam to the optical fiber 24 in a direction Z as shown by thearrow, and by generating an acoustic wave in the transducer 10, thedevice operates as an output device and an output beam exits through theglass block 30. In this condition, a photographic plate 52 for theproduction of a hologram is placed as shown and a laser beam is producedby a laser generator 60 through a lens 56 while the slit 38 is removed.As is well known in the art, the laser beam and the output light beamcreate an interference pattern on the photographic plate 52. The exposedplate is then developed to obtain a hologram, which, in the input mode,is positioned at the same location (52). The laser beam is againemanated from the generator 60 through the lens 56 mounted on supportmeans 58, and the slit 38 is positioned as shown in FIG. 13. With thisarrangement an input light beam is generated which propagates exactly inreverse to the output light beam, and the input light beam can be fedinto the optical fiber by appropriately energizing the transducer 10 asdeveloped above. The relation between the incident input light beam andthe guided mode is the same as between the output light beam and theguided mode. By varying the width or location of the slit 38 and/or thefrequency of the acoustic wave, the incident light beam can be fed intothe optical fiber either in a particular mode or in all the modestherein.

The input resolution will be the same as that of the output mode. Theinput efficiency will be lowered slightly as compared with the outputefficiency due to low diffraction efficiency of the hologram.

As has been explained in the foregoing, the input and output devices foran optical fiber according to the present invention inject in or extractout the light beam by using Bragg reflection, and have the significantadvantage that the light can be read in or out without cutting ordamaging the optical fiber. Furthermore, no rigid or permanentconnections are necessary between the quartz block and the opticalfiber, whereby the device is portable and can be arranged freely at anydesired location for use in the input or output modes.

When the device is used in an output mode, it is possible to extracteach of the propagated light modes separately so that complete modeanalysis is possible at any desired location on the optical fiber. Thedevice can also be used as an optical monitor, whereby the transmissioncharacteristics of the optical fiber may be measured during themanufacture thereof. Such measurements may be used in a feedback loopfor controlling the manufacturing of the fiber.

When using the device in an input mode the light can be injected at anydesired location of the optical fiber, whereby the device may be usedfor the detection of faults or trouble points, or as a transmissiontester for the connection or adjustment of the optical fiber. By acombined use of the input and output modes, a more effectivetransmission tester can be realized.

Separate input and output devices may be mounted at an arbitrary span onan optical fiber, whereby it is possible to send a signal from the inputdevice to the output device to form a communication link without cuttingthe fiber.

While the principles of this invention are not limited to thepropagation of any particular type or mode of communications signal,significant promise and potential would appear to lie in the field ofwideband digital communications for telephone subscriber systems.

What is claimed is:
 1. An acoustical output device for an optical, lightconducting fiber, comprising: a quartz block having a semi-circulargroove therein of a radius substantially equal to that of an elongated,circular optical fiber, said block having an inclined upper surface, anacoustic transducer secured to said upper surface, a glass block havinga refractive index higher than that of the optical fiber and having asemi-circular groove therein of a radius substantially equal to that ofthe optical fiber, said glass block having an inclined bottom surface,the quartz block and the glass block being arranged to embrace theoptical fiber within their respective grooves at a location where anoutput is desired, the inclined surfaces of the blocks being oppositeeach other with the narrower ends thereof together, an opticallytransparent sheet arranged between the groove of the glass block and theoptical fiber, a liquid, such as water, provided between the opticalfiber and the quartz block, such liquid having a low absorption loss foran acoustic wave and an acoustic impedance close to that of the quartzblock to thereby cause a small reflection loss for an acoustic wave, theacoustic transducer being arranged to generate an acoustic wave havingan inclined wave front with respect to the axis of the optical fiberalong which light is propagated in guided modes to thereby cause lightto be reflected by Bragg's condition to a radiation mode, and an opticaloutput system aligned with a beam of output light emanated from theoptical fiber, the optical output system being separably arranged fromthe glass block.
 2. An output device for an optical fiber as claimed inclaim 1, wherein the optical fiber is a single mode fiber, and forobtaining an output mode whose propagation constant is β, theinclination angle α of the acoustic transducer with respect to the axisof the optical fiber is selected to satisfy the condition:

    β/k.sub.2 cosα ≦ 1,

wherein k₂ is the wave number of a planar wave in a clad of the opticalfiber, and the frequency f of the acoustic transducer is given by:##EQU10## wherein W is the velocity of the acoustic wave.
 3. An outputdevice for an optical fiber as claimed in claim 1, wherein the acoustictransducer has a curved surface to efficiently concentrate acousticpower onto the optical fiber.
 4. An output device for an optical fiberas claimed in claim 1, wherein said sheet is polyethylene.
 5. An outputdevice for an optical fiber as claimed in claim 1, wherein the opticalfiber is a multi-mode fiber,α is the inclination angle of the acoustictransducer with respect to the axis of the optical fiber, f is thefrequency of the acoustic wave generated by the acoustic transducer, k₁is the wave number of a planar light wave in a core of the opticalfiber, W is the velocity of the acoustic wave, θ is the propagationangle of guided modes of light propagating in the optical fiber, andθ_(m) is the propagation angle of the highest order guided mode,andwherein α and f are chosen to satisfy the following conditions:##EQU11##
 6. An output device for an optical fiber as claimed in claim5, wherein the position of the optical output system is fixed, α is setat a value satisfying the condition:

    α ≧ θ.sub.m,

f is set at a value satisfying the condition: f=(k₁ W/π)sin(α-θ), and θis varied between 0 and θ_(m), whereby all of the guided modespropagating in the fiber are extracted.
 7. An output device for anoptical fiber as claimed in claim 5, wherein the acoustic frequency f isfixed in a range of

    f ≦(k.sub.1 W/2) sin .sup.θ m/2,

and the inclination angle α is fixed with respect to said value of f tosatisfy the condition: ##EQU12##whereby by moving the optical outputsystem all of the guided modes propagating in the optical fiber areextracted.
 8. An acoustical input device for an optical, lightconducting fiber, comprising: a quartz block having a semi-circulargroove therein of a radius substantially equal to that of an elongated,circular optical fiber, said block having an inclined upper surface, anacoustic transducer secured to said upper surface, a glass block havinga refractive index higher than that of the optical fiber and having asemi-circular groove therein of a radius substantially equal to that ofthe optical fiber, said glass block having an inclined bottom surface,the quartz block and the glass block being arranged to embrace theoptical fiber within their respective grooves where an input is desired,the inclined surfaces of the blocks being opposite each other with thenarrower ends thereof together, an optically transparent sheet arrangedbetween the groove of the glass block and the optical fiber, a liquid,such as water, provided between the optical fiber and the quartz block,such liquid having a low absorption loss for an acoustic wave and anacoustic impedance close to that of the quartz block to thereby cause asmall reflection loss for an acoustic wave, an optical input systemaligned with a beam of input light and being arranged separably from theglass block, the acoustic transducer being arranged to generate anacoustic wave having an inclined wave front with respect to the axis ofthe optical fiber, such acoustic wave thereby causing a beam of inputlight to be injected by Bragg's condition into the optical fiber andpropagated therein in a guided mode.
 9. An input device for an opticalfiber as claimed in claim 8, wherein the optical fiber is a single modefiber, and for obtaining an input mode whose propagation constant is β,the inclination angle α of the acoustic transducer with respect to theaxis of the optical fiber is selected to satisfy the condition:

    β/k.sub.2 cosα ≦ 1,

wherein k₂ is the wave number of a planar wave in a clad of the opticalfiber, and the frequency f of the acoustic transducer is given by:##EQU13## wherein W is the velocity of the acoustic wave.
 10. An inputdevice for an optical fiber as claimed in claim 8, wherein the acoustictransducer has a curved surface to efficiently concentrate acousticpower onto the optical fiber.
 11. An input device for an optical fiberas claimed in claim 8, wherein said sheet is polyethylene.
 12. An inputdevice for an optical fiber as claimed in claim 8, wherein the opticalfiber is a multi-mode fiber,α is the inclination angle of the acoustictransducer with respect to the axis of the optical fiber, f is thefrequency of the acoustic wave generated by the acoustic transducer, k₁is the wave number of a planar light wave in a core of the opticalfiber, W is the velocity of the acoustic wave, θ is the propagationangle of guided modes of light propagating in the optical fiber, andθ_(m) is the propagation angle of the highest order guided mode,andwherein α and f are chosen to satisfy the following conditions.##EQU14##
 13. An input device for an optical fiber as claimed in claim12, wherein the position of the optical input system is fixed, α is setat a value satisfying the condition:

    α ≧ θ.sub.m,

f is set at a value satisfying the condition: f=k₁ W/πsin(α-θ), and θ isvaried between 0 and θ_(m), whereby the input light is injected into theoptical fiber in all of the guided modes propagating therein.
 14. Aninput device for an optical fiber as claimed in claim 12, wherein theacoustic frequency f is fixed in a range of:

    f ≧ (k.sub.1 W/2) sin .sup.θ m/2,

and the inclination angle α is fixed with respect to said value of f tosatisfy the condition: ##EQU15##whereby by moving the optical inputsystem the light is the guided modes propagating in the fiber.